<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
</head>

<body>
    <div id="app">
        <button @click="count++">修改count</button>
        <button @click="unWatch">取消</button>
        <p>{{count}}</p>
    </div>
    <script>
        Vue.config.productionTip = false;

        const vm = new Vue({
            el: "#app",
            data() {
                return {
                    count: 0,
                }
            },
            methods: {
                /* 
      
                  临时存放一个回调函数给unWacth,在将来$watch监听后，把返回取消监听函数赋值过来
      
                */
                unWatch: () => { }
            }
        })

        //使用$watch可以在外部书写监听
        //$watch返回一个unWatch的方法，用来取消当前的监听

        vm.unWatch = vm.$watch(
            "count",
            function () {
                console.log("count改变了");
                console.log(this);
            },
            {
                deep: true,
                immediate: true,
            }
        )
    </script>
</body>

</html>